Bulk acoustic wave resonator and circuit comprising same

ABSTRACT

The invention relates to a resonator operating with bulk acoustic waves (BAW resonator, BAW=Bulk Acoustic Wave) and band-pass filters constructed of such resonators. To increase the edge steepness of the transmission band of a BAW band-pass filter, the invention proposes reducing the effective coupling of a BAW resonator by using the connection in parallel of a BAW resonator and a capacitor instead of only one resonator. In addition, to increase the edge steepness of the transmission band, the use of a connection of coupled BAW resonators in the serial branch of a filter circuit with another resonator or resonator stack in the parallel branch of the filter circuit is proposed, the additional resonator or resonator stack being connected to the center electrode of the resonator stack specified initially.

TECHNICAL FIELD

This patent application describes a resonator operating with bulkacoustic waves (or FBAR, Thin Film Bulk Acoustic Wave Resonator), alsoknown as BAW resonator (Bulk Acoustic Wave Resonator), as well as acircuit constructed of such resonators.

BACKGROUND

BAW resonators are suitable, in particular, for band-pass high-frequencyfilters in modern filter technology, and can be used, for example, inmobile communication devices.

A resonator operating with bulk acoustic waves has a piezoelectric layerthat is disposed between two metal layers (electrodes). A sequence oflayers can also be used instead of only one piezoelectric layer. Thelayers are deposited consecutively on a substrate and structured intoresonators, that are electrically connected to one another and togethercan constitute, for example, a filter circuit especially a band-passfilter. Such a band-pass filter can also be used together with anotherfilter in a duplexer.

FIG. 1 shows the equivalent circuit diagram of a BAW resonator. Outsidea frequency range surrounding the resonant frequency, the resonator ischaracterized by a static capacitor C₀ and, in proximity to the resonantfrequency, by the series connection of a resistor R_(m), a capacitorC_(m) and an inductive resistor L_(m). The static capacitor isessentially defined by the resonator surface area and the thickness ofthe piezoelectric layer. The resistor R_(m) describes losses in theresonator, while the capacitor C_(m) and the inductive resistor L_(m)determine the resonant frequency

$f_{r} = {\frac{1}{2\;\pi\sqrt{L_{m}C_{m}}}.}$

The ratio C_(m)/C_(o) determines the coupling of the resonator. Thecoupling coefficient k of the resonator is linked to the resonantfrequency f_(r) and the antiresonant frequency f_(a):

${k^{2} = \frac{f_{a}^{2} - f_{r}^{2}}{f_{a}^{2}}},$wherein f_(o)=f_(m)√{square root over (1+C_(m)/C_(o))}.

A band-pass filter is characterized by a transfer function that has, inparticular, a pass band and several stop bands. The pass band is, inturn, characterized by its bandwidth, the insertion attenuation in thepass band and the edge steepness at the edge of the pass band.

Two BAW resonators SR1 and SR2 (as depicted schematically in FIG. 2) canbe acoustically coupled with one another if, for example, they arearranged in a stack one on top of the other. In this connection, theresonators form a series connection between a port P1 and a port P2,e.g., in a stacked-crystal arrangement, in which two resonators share acommon electrode, that is connected to ground (see FIG. 3), or arearranged in a coupled-resonator arrangement, in which a coupling layerKS is arranged between the upper electrode E2 of the lower resonator andthe lower electrode E3 of the upper resonator, and said electrodes areconnected to ground (see FIG. 4). A first resonator in FIG. 3 comprisesa piezoelectric layer PS1, that is arranged between two electrodes E1and E2, and an acoustic mirror AS arranged below the electrode E1, saidacoustic mirror resting on a carrier substrate TS. Above the firstresonator, a second resonator is arranged that comprises a piezoelectriclayer PS2, which is arranged between the electrode E2 and an electrodeE3. Electrode E1 is connected to port P1, electrode E3 to port P2 andelectrode E2 to ground.

The layer system shown in FIG. 4 includes a first resonator arranged ona carrier substrate TS, a coupling layer KS disposed above it and asecond resonator arranged above the coupling layer KS. The firstresonator is arranged as described in FIG. 3 and is connected betweenport P1 and ground. The second resonator contains (from bottom to top)two electrodes E3 and E4 and a piezoelectric layer PS2 arranged betweensaid electrodes, the second resonator being connected between port P2and ground. The coupling layer KS arranged between said resonatorsprovides for acoustic coupling between these resonators.

Filters constructed of acoustically coupled resonators are characterizedby a high stop band suppression. However, the edge steepness and, withit, the adjacent channel selectivity are comparatively low, due to theabsence of defined pole positions in proximity to the pass band.

BAW resonators can be connected in a ladder-type or a lattice-typeconstruction. The advantage of the lattice-type arrangement of theresonators in a band-pass filter is that the selection of such a filterin stop band areas well outside the pass band is very good, ranging, forexample, between −40 and −60 dB. The disadvantage of this filterarrangement includes a low edge steepness of the pass band. For thisreason, it may be difficult, in this type of filter arrangement, toachieve sufficient attenuation of the signal in the stop bands inproximity to the pass band.

Considerable edge steepness is required in some applications. In thecase of duplexers that are suitable for the PCS telecommunicationsstandard, for example, a decline in the transmission function from ca.−3 dB to significantly below −40 dB within a frequency range of only 20MHz must be guaranteed. Previously known band-pass filters that areconstructed of BAW resonators may not satisfy such requirements, due toadditional frequency shifts in the edges in response to temperaturechange or as a result of existing production tolerances (which, in thecase of a filter operating at ca. 2 GHz and having BAW resonators thatcontain a piezoelectric layer of ALN, can amount to several MHz).

It is known, from the reference EP 0949756 A2, that a series connectionof stacked resonators acoustically coupled with one another, as well asadditional resonators instead of only one resonator in a filter circuit,improves edge steepness in the transmission band of the filter. Thedisadvantage of this solution, however, is that it requires a great dealof space.

SUMMARY

This patent application describes a resonator operating with bulkacoustic waves (also known as BAW resonator—Bulk Acoustic WaveResonator—or FBAR—Thin Film Bulk Acoustic Wave Resonator), which isconstructed of a sequence of layers containing the following layers: alower layer region that comprises a first electrode, an upper layerregion that comprises a second electrode and, between the two, apiezoelectric layer. A capacitor is connected in parallel or in seriesto the resonator.

The parallel connection of a BAW resonator and a capacitor C_(a) insteadof a non-connected resonator reduces the effective coupling of the BAWresonator (that is, the distance between the resonant and antiresonantfrequency of the resonator), in that the effective static capacitanceC′₀ is increased, C′₀=C₀+C₀. In this connection, the resonant frequencyf′_(r) of the new circuit (series resonance, or the resonant frequencyof the series resonant circuit formed by C_(m), L_(m) and R_(m)) remainsunchanged relative to the resonant frequency f_(r) of the(non-connected) resonator, f′_(r)=f_(r). In contrast, the antiresonantfrequency f_(o)=f′_(m)√{square root over (1+C_(m)/C′_(o))} (parallelresonance, or the resonant frequency of the parallel resonant circuitformed by C′₀, C_(m), L_(m) and R_(m)) is lower than the antiresonantfrequency f_(o)=f_(m)√{square root over (1+C_(m)/C_(o))} (parallelresonance, or the resonant frequency of the parallel resonant circuitformed by C₀, C_(m), L_(m) and R_(m)) of the (non-connected) resonator.As a result, the edge steepness of a band-pass filter comprising suchBAW resonators is increased.

The series connection of a BAW resonator and a capacitor C_(a) insteadof a non-connected resonator reduces the effective coupling of a BAWresonator (that is, the distance between the resonant and theantiresonant frequency of the resonator). In the connection, theantiresonant frequency f′_(a) of the circuit (parallel resonance, or theresonant frequency of the parallel resonant circuit formed by C₀, C_(m),L_(m) and R_(m)) remains unchanged relative to the antiresonantfrequency f_(a) of the resonator, f′_(a)=f_(a). In contrast, theresonant frequency f′_(m)=f_(m)√{square root over(1+C_(m)(C_(o)+C_(o)))} (series resonance, or the resonant frequency ofthe series resonant circuit formed by C₀, C_(m), L_(m) and R_(m)) of thecircuit is higher than the resonant frequency f_(r) (series resonance,or the resonant frequency of the serial resonant circuit formed byC_(m), L_(m) and R_(m)) of the resonator. As a result, the edgesteepness of a band-pass filter comprising such BAW resonators isincreased.

In an embodiment, the resonator is arranged on a carrier substrate. Itis also possible to arrange the resonator over an air gap provided inthe carrier substrate.

The first and the second electrode may include an electricallyconductive material, such as a metal or a metal alloy.

The piezoelectric layer may include AlN, but can include anothermaterial with piezoelectric properties (such as ZnO). It is alsopossible that the piezoelectric layer comprises a plurality of adjacentor separated, identical or different layers with piezoelectricproperties.

It is possible that the first and/or the second electrode has amultilayer structure comprised of two or more adjacent layers ofdifferent materials. It is also possible that the piezoelectric layer inthe resonator comprises two or more adjacent or separated layers ofdifferent materials.

It is possible that, additionally, a layer resistant to dielectricdischarge is arranged between the first and the second electrode, wherethe layer protects the resonator against electric arcing between theelectrodes.

The connection of a capacitor in parallel to a BAW resonator can beaccomplished in a filter constructed, for example, in a ladder-typeconstruction, in a lattice-type construction or as an SCF (StackedCrystal Filter), as well as of any combination of BAW resonators.

It is possible to provide for the connection of a capacitor in parallelto a BAW resonator in only one series branch or in a plurality of seriesbranches of a filter. It is also possible to provide for the connectionof a capacitor in parallel to a BAW resonator in only one parallelbranch or a plurality of parallel branches of a filter. In a furtherembodiment, it is possible that the connection of a capacitor inparallel to a BAW resonator be provided in at least one series branch orin at least one parallel branch of the filter.

In embodiments, the value of the capacitor connected in parallel to aBAW resonator may be between 0.1 and 10 pF.

It is advantageous when the coupling of the resonator is reduced only inthe series branches or only in the parallel branches of a filter or aduplexer by the parallel connection of the corresponding capacitors.

It is possible to implement the capacitor connected in parallel to a BAWresonator by connecting a discrete capacitor in parallel to the BAWresonator. Another possibility is to realize such a capacitor in thecarrier substrate by structured metal layers. It is also possible toarrange an additional dielectric layer between the electrodes of the BAWresonator to increase the capacitance of the BAW resonator. Thisdielectric layer can be arranged between the piezoelectric layer and oneof the electrodes or between two piezoelectric layers.

The parasitic capacitance of the respective resonator can also bedeliberately selected to be as large as possible, for example byenlarging the electrode surface to improve the edge steepness of thefilter constructed of such resonators. Other implementations not citedhere are also possible.

It is possible that the lower and/or upper layer region of the resonatormay include one or more layers. It is also possible that an acousticmirror is realized in the lower and/or in the upper layer region, wherethe mirror comprises at least two alternating layers having differentacoustic impedance.

The acoustic mirror may comprise alternating layers, each having a highand a low acoustic impedance, each of their layer thicknesses comprisingapproximately a quarter wavelength of the acoustic main mode (relativeto the velocity of expansion of the acoustic wave in the respectivematerial). The acoustic mirror thus provides one and/or a plurality ofboundary surfaces, that, at the resonant frequency, reflect the acousticwave back into the resonator and prevent the wave from escaping in thedirection of the carrier substrate.

In a further advantageous embodiment, one of the layers of the acousticmirror can simultaneously constitute one of said electrodes.

The use of a BAW resonator with a capacitor connected in parallel in thecircuit of a band-pass filter increases the edge steepness of thetransmission band of the band-pass filter. As a result, the attenuationof the signal is increased in the stop bands in proximity to the passband. This is advantageous in the case of realization of a duplexercircuit having such a band-pass filter.

Another embodiment includes an electric circuit containing a resonatorstack that comprises at least two resonators arranged one on top of theother and operating with bulk acoustic waves and at least one additionalresonator or resonator stack having BAW resonators. Each of theresonators operating with bulk acoustic waves comprises a lowerelectrode, an upper electrode and a piezoelectric layer arranged betweenthe two. In this connection, the resonators arranged one on top of theother in the resonator stack form a series circuit, e.g., in a stackedcrystal arrangement (when both resonators have a shared electrode) or acoupled resonator arrangement (when a coupling layer is provided betweenthe upper electrode of the lower resonator and the lower electrode ofthe upper resonator).

The upper electrode of the lower resonator operating with bulk acousticwaves and the lower electrode of the upper resonator operating with bulkacoustic waves, said electrodes being arranged in the resonator stack,are electrically connected here to one of the electrodes of at least oneadditional resonator or resonator stack.

The connection can be viewed as a basic element of a ladder-typearrangement or (in the case of a suitable connection) of a lattice-typearrangement of individual resonators, at least two of the resonatorsbeing acoustically coupled with one another and arranged one on top ofthe other. It is possible that two BAW resonators arranged one on top ofthe other in a stack realize here two series resonators or parallelresonators of the ladder-type arrangement or of the lattice-typearrangement. It is also possible that two BAW resonators arranged one ontop of the other in a stack realize one series resonator and oneparallel resonator of the ladder-type arrangement or the lattice-typearrangement.

A coupling layer may be provided between the upper electrode of thelower resonator operating with bulk acoustic waves and the lowerelectrode of the upper resonator operating with bulk acoustic waves,said electrode being arranged in the resonator stack.

The at least one additional resonator can, for example, be a resonatorwith bulk acoustic waves, a resonator operating with surface acousticwaves, an LC resonator or a resonator stack as specified above.

The second electrode of the at least one additional resonator, saidelectrode not being connected to the resonators arranged one on top ofthe other in the resonator stack, can be connected to ground or to asubsequent resonator and/or to a resonator stack not yet specified.

The circuit represents an advantageous combination of different filterarrangements, such as the arrangement of the resonators stacked one ontop of the other and acoustically coupled with one another, as well as aladder-type arrangement and/or a lattice-type arrangement. The transferfunction of a filter whose basic elements realize the circuit, ascompared with the transfer function of a filter constructed of resonatorstacks known in the art, exhibits significantly steeper edges in thepass band of the filter. This results in good adjacent channelselectivity of the filter.

The circuit that includes a resonator stack and a resonator electricallyconnected to it as specified above may comprise a basic element of afilter.

It is possible that a plurality of parallel resonators, each of which isarranged in a parallel branch of different basic elements electricallyconnected to one another, are acoustically connected to one anotherand/or arranged one on top of the other. It is also possible that,instead of only one resonator being realized in the parallel branch(parallel resonator) of a basic element of the circuit, two (e.g.,coupled with one another) parallel resonators connected in series or inparallel are realized.

It is also possible that more than only two series resonators arearranged one on top of the other and/or acoustically coupled with oneanother.

The basic elements of the described above can be combined with oneanother in any manner.

In the following, embodiments are explained in greater detail on thebasis of figures that are schematic and, therefore, not true to scale.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an equivalent circuit diagram of a BAW resonator.

FIG. 2 shows the circuit diagram of a resonator stack.

FIG. 3 shows a resonator stack with acoustically coupled BAW resonatorsin schematic cross-section (state of the art).

FIG. 4 shows another example of a resonator stack with acousticallycoupled BAW resonators and a coupling layer in schematic cross-section(state of the art).

FIG. 5 a shows an equivalent circuit diagram of a BAW resonator with acapacitor connected in parallel.

FIG. 5 b shows an equivalent circuit diagram of a BAW resonator with acapacitor connected in series.

FIG. 6 a shows a basic element of a filter realized in ladder-typeconstruction with a capacitor connected in parallel to a BAW resonatorin the series branch.

FIG. 6 b shows the transfer function of a filter realized in ladder-typeconstruction without and with a capacitor connected in parallel to a BAWresonator in the series branch.

FIG. 7 shows a basic element of a filter realized in ladder-typeconstruction with a capacitor connected in parallel to a BAW resonatorin the parallel branch.

FIG. 8 a shows an embodiment of a filter realized in ladder-typeconstruction with capacitors connected in parallel to BAW resonators inthe serial branches.

FIG. 8 b shows the transfer function of a filter realized inlattice-type construction without and with a capacitor connected inparallel to a BAW resonator in the series branch.

FIG. 9 shows an embodiment of a filter realized in lattice-typeconstruction with capacitors connected in parallel to BAW resonators inthe parallel branches.

FIG. 10 shows a connection of a resonator stack in the series branch andof an additional BAW resonator in the parallel branch, in circuitdiagram (a) and in schematic cross-section (b), respectively.

FIGS. 10 c and 10 d show LC filter arrangements.

FIG. 11 shows an advantageous embodiment of a connection of a resonatorstack and of an additional BAW resonator in schematic cross-section.

FIG. 12 shows a connection of a resonator stack in the series branch andof an additional resonator stack in the parallel branch, in circuitdiagram (a) and in schematic cross-section (b), respectively.

DETAILED DESCRIPTION

FIGS. 1 to 4 have already been discussed earlier. FIG. 5 a shows anequivalent circuit diagram of a BAW resonator with a capacitor C_(a)connected in parallel to it. Outside the resonant frequency range, theresonator includes a static capacitor C₀ and, in proximity to theresonant frequency, by a resistor R_(m), a capacitor C_(m) and aninductive resistor L_(m). The resistor R_(m) describes losses in theresonator, while the capacitor C_(m) and the inductive resistor L_(m)determine the resonant frequency. The ratio C_(m)/C₀ determines thecoupling of the resonator. The addition of a capacitor C_(a) connectedin parallel to the resonator results in reduction of the effectivecoupling of the resonator determined by C_(m)/(C₀+C_(a)), instead ofC_(m)/C₀.

FIG. 5 b shows an equivalent circuit diagram of a BAW resonator with acapacitor C_(a) connected in series to it.

An example of a connection of two BAW resonators RA and RB inladder-type construction and a capacitor C_(a) connected in parallel toone of the resonators is shown in FIG. 6 a. Resonator RA is arranged ina series branch and resonator RB in a parallel branch of the circuit.Two resonators connected in this manner represent, for example, a basicelement of a ladder-type filter known in the art.

In FIG. 6 a, the capacitor C_(a) is integrated in the series branch ofthe circuit. In this connection, it is connected in parallel to theseries resonator RA, as a result of which the steepness of the rightedge of the transfer function in the pass band can be controlled orincreased. Such a basic element can be used, for example, in thetransmission filter (Tx filter) of a duplexer, especially a PCSduplexer.

FIG. 6 b shows the transfer function S21 of a filter realized inladder-type construction without and with a capacitor connected inparallel to a BAW resonator in the series branch. The transfer functionof the filter constructed of BAW resonators in the ladder-typeconstruction known in the art is indicated by a dashed line 11. Thetransfer function of the filter in ladder-type construction with acapacitor connected in parallel to a BAW resonator in the series branchis indicated by a continuous line 12, wherein the transfer function, inthis case, has a steeper right edge of the pass band.

In FIG. 7, the capacitor C_(a) is integrated in the parallel branch ofthe circuit. It is connected in parallel to the parallel resonator RB,as a result of which the steepness of the left edge of the transferfunction in the pass band can be controlled and/or increased. Such abasic element can be used, for example, in the reception filter (Rxfilter) of a duplexer, especially a PCS duplexer.

The capacitor C_(a) can be arranged on a carrier substrate, togetherwith the BAW resonator. The capacitor C_(a) can also constitute adiscrete component with external electrodes, said component beingelectrically connected to the BAW resonator as described above.

It is also possible that the capacitor C_(a) is realized in themetallized layers of the (multilayer) carrier substrate and, asdescribed above, is electrically connected to the BAW resonator by, forexample, feedthroughs, bump connectors or bond wires.

An example of a connection of two BAW resonators RA and RB inlattice-type construction and a capacitor C_(a) connected in parallel toone of said resonators is shown in FIG. 8 a. A resonator RA is arrangedin a series branch, and a resonator RB in a parallel branch of thecircuit. FIG. 8 a shows two pairs of resonators that are connected inthis manner and this constitutes, for example, a basic element of afilter realized in lattice-type construction.

In FIG. 8 a, two capacitors C_(a) are each integrated in a series branchof the circuit. They are each connected in parallel to the correspondingseries resonator RA, as a result of which the steepness of the rightedge of the transfer function in the pass band can be controlled and/orincreased. Such a basic element can be used, for example, in thetransmission filter (Tx filter) of a duplexer, especially a PCSduplexer.

FIG. 8 b shows the transfer function S21 of a filter realized inlattice-type construction without and with a capacitor connected inparallel to a BAW resonator in the series branch. The transfer functionof the filter constructed of BAW resonators in the lattice-typeconstruction known in the art is indicated by a dashed line 11. Thetransfer function of the filter in lattice-type construction with acapacitor connected in parallel to a BAW resonator in the series branchis indicated by a continuous line 12, wherein the transfer function, inthis case, has a steeper right edge of the pass band.

In FIG. 9, two capacitors C_(a) are each integrated in a parallel branchof the circuit. They are each connected in parallel to the parallelresonator RB, as a result of which the steepness of the left edge of thetransfer function in the pass band can be controlled or increased. Sucha basic element can be used, for example, in the reception filter (Rxfilter) of a duplexer, especially a PCS duplexer.

FIG. 10 a shows the circuit diagram of a connection of a resonatorstack, that comprises the BAW resonators SR1 and SR2, in the seriesbranch, and of an additional BAW resonator PR in the parallel branch.The resonator stack is connected between ports P1 and P2. An example ofa realization of such a circuit is shown in schematic cross-section inFIG. 10 b. The resonator stack comprises the piezoelectric layer PS1,that is arranged between two electrodes E1 and E2 (center electrode).The piezoelectric layer PS2 is arranged above them. An electrode E4connected to the port 2 lies on the piezoelectric layer PS2. The port P1is electrically connected to the electrode E1. The layer sequence E1,PS1 and E2 realizes, for example, the resonator SR1 in accordance withFIG. 10 a. The layer sequence E2, PS2 and E4 realizes, for example, theresonator SR2 in accordance with FIG. 10 a. Here, the resonator PR inthe parallel branch of the circuit according to FIG. 10 a is realized bythe layer sequence E6 (electrode), PS3 (piezoelectric layer) and E5(electrode), the electrode E5 being electrically connected to the centerelectrode E2. In this embodiment, the electrode E6 is connected toground. It is also possible that it is connected to another circuit notshown here.

FIGS. 10 c and 10 d show series and parallel LC resonator arrangements,respectively. LC resonators that are not part of a BAW stack (e.g., theparallel resonator PR of FIG. 10 a) may be configured as shown in FIG.10 c or 10 d.

FIG. 11 shows, in schematic cross-section, an embodiment of a resonatorstack and an additional BAW resonator. The resonator stack includes,from bottom to top, a first electrode E1, a first piezoelectric layerPS1, a second electrode E2, a coupling layer KS1, a third electrode E3,a second piezoelectric layer PS2 and a fourth electrode E4. Theresonator stack forms two resonators arranged one on top of the otherand coupled with one another by the coupling layer (corresponding to SR1and SR2 in FIG. 10 a), and is connected between ports P1 and P2. Theparallel branch of the circuit is formed by an additional resonator,that includes a third piezoelectric layer PS3 and electrodes E5 and E6surrounding it. Electrodes E2 and E3 are connected to electrode E5.Here, electrode E6 is connected to ground. It is also possible that itbe connected to another circuit not shown here.

FIG. 12 a shows the circuit diagram of a connection of a resonator stackin the series branch and another resonator stack in the parallel branchbetween ports P1 and P2. The first resonator stack includes tworesonators SR1 and SR2 connected in series. The second resonator stackincludes two resonators PR1 and PR2 connected in series. An example of arealization of this circuit is shown in schematic cross-section in FIG.12 b. The first resonator stack is constructed as shown in FIG. 10 b.The second resonator stack includes, from bottom to top, an electrode E6(connected to ground, for example), a piezoelectric layer PS3, a centerelectrode E5, that is electrically connected to electrode E2 of thefirst resonator stack, a piezoelectric layer PS4 and an electrode E7(connected to ground, for example).

Though not specifically shown in the figure, the (lower) resonators are,in this case, also arranged on a carrier substrate, where an air gap oran acoustic mirror is provided, in each case, between the carriersubstrate and resonator.

In the interest of clarity, only a few embodiments are described;however, the claims are not limited to these. Other variations arepossible, especially in light of the possible combinations of the basicelements and arrangements presented above, as well as the number oflayers in said layer regions of the resonator. The claims are notlimited to a specific frequency range or a specific scope ofapplication. Each of the layers of the resonator (e.g., thepiezoelectric layer or the electrode) can have a multilayer structure.The resonator can also contain a plurality of (e.g., possiblynon-adjacent) piezoelectric layers or more than only two electrodes.

The electrical connections (including the connections to ground) in theexemplary embodiments described can contain discrete elements, such asinductive resistors, capacitors, delay lines or matching networks.

1. A filter comprising resonators for use with bulk acoustic waves, eachof the resonators for use with bulk acoustic waves comprising: a lowerlayer region comprising a first electrode; an upper layer regioncomprising a second electrode; and a piezoelectric layer between thefirst electrode and the second electrode; wherein two of the resonatorsare in a stacked crystal filter arrangement, the two of the resonatorscomprising two bulk acoustic wave resonators, the stacked crystal filterarrangement of the two bulk acoustic wave resonators comprising:piezoelectric layers between an upper electrode in the stacked crystalfilter arrangement and a lower electrode in the stacked crystal filterarrangement; and a shared electrode among the piezoelectric layers inthe stacked crystal filter arrangement; wherein an additional resonatoris connected to the stacked crystal filter arrangement so that acombination of the two resonators and the additional resonator form anelement of a lattice-type filter or a ladder-type filter, the additionalresonator comprising a bulk acoustic wave resonator or aninductive-capacitive (LC) resonator; a capacitor in series or inparallel with one of the two resonators in the stacked crystal filterarrangement, and a multilayer substrate, wherein the capacitor isintegrated into the multilayer substrate, the capacitor comprisingstructured metal layers within the multilayer substrate.
 2. The filterof claim 1, wherein the additional resonator comprises at least onepassive inductive component and at least one passive capacitivecomponent.
 3. The filter of claim 1, wherein each of the upper layerregion and the lower layer region comprises a plurality of layers. 4.The filter of claim 3, wherein a plurality of layers in the upper layerregion comprises layers that include different materials, and aplurality of layers in the lower layer region comprises layers thatinclude different materials.
 5. The filter of claim 1, wherein at leastone of the upper layer region and the lower layer region comprises anacoustic mirror, the acoustic mirror comprising at least two alternatinglayers having different acoustic impedances.
 6. The filter of claim 5,wherein at least one layer of the acoustic mirror comprises an electrodelayer.
 7. The filter of claim 1, wherein there is an air gap between atleast one of the resonators and the multilayer substrate.
 8. The filterof claim 1, wherein the additional resonator is an LC resonator.
 9. Aduplexer comprising a filter according to claim
 8. 10. A duplexercomprising a filter according to claim
 8. 11. The filter of claim 1,wherein, for at least one of the resonators for use with bulk acousticwaves, an upper layer region and a lower layer region comprises aplurality of layers.
 12. The filter of claim 11, wherein a plurality oflayers in each upper layer region comprises layers that includedifferent materials, and a plurality of layers in each lower layerregion comprises layers that include different materials.
 13. The filterof claim 11, wherein there is an air gap between at least one of theresonators and the multilayer substrate.
 14. The filter of claim 1,wherein each upper layer region and each lower layer region comprises anacoustic mirror, each acoustic mirror comprising at least twoalternating layers having different acoustic impedances.
 15. The filterof claim 14, wherein at least one layer of each acoustic mirrorcomprises an electrode layer.
 16. An electrical circuit comprising: asubstrate; a stack of resonators; an acoustic mirror between thesubstrate and the stack of resonators; wherein the stack of resonatorscomprises: first resonators that operate with bulk acoustic waves, thefirst resonators comprising an upper resonator and a lower resonator,the upper resonator and the lower resonator comprising upper and lowerelectrodes; and a coupling layer between an upper electrode of the lowerresonator and a lower electrode of the upper resonator; a secondresonator comprising electrodes; wherein the upper electrode of thelower resonator and the lower electrode of the upper resonator areelectrically connected to an electrode of the second resonator; whereinthe electrical circuit further comprises a capacitor in parallel with atleast one of the resonators or in series with at least one of theresonators; wherein the substrate comprises a multilayer substrate; andwherein the capacitor is integrated into the multilayer substrate thecapacitor comprising structured metal layers within the multilayersubstrate.
 17. The electrical circuit of claim 16, wherein an electrodeof the second resonator is connected to ground.
 18. The electricalcircuit of claim 16, wherein the second resonator comprises a singleresonator, the single resonator comprising a lower electrode, an upperelectrode, and a piezoelectric layer between the upper electrode and thelower electrode.